1. Field of the Invention
The present invention relates to a formed SiC product which possesses a high purity, exhibits an excellent heat resistance as well as mechanical strengths and, in particular, exhibits an excellent lack of transparency to light, making it suitable for use in a variety of heat resistant components such as shielding materials, equalizing rings, and others used in heat treatment equipment in semiconductor manufacturing facilities, or in dummy wafers and other components used in diffusion furnaces, etching equipment, CVD equipment, and the like in semiconductor manufacturing facilities. The present invention also relates to a method for manufacturing such a formed SiC product.
2. Description of the Background Art
Silicon carbide (SiC) possesses excellent material characteristics such as heat resistance, corrosion resistance, and mechanical strength, and therefore is advantageously utilized as a material for various industrial applications. In particular, a formed SiC product manufactured by utilizing a CVD method (i.e. chemical vapor deposition method), hereinafter to be referred to as a "CVD-formed SiC product", is favorably used in application fields where a high purity is required, including use in various components for manufacturing semiconductors, because of its high density and high purity.
Such a CVD-formed SiC product is obtained by causing feed gases to react in the vapor phase to deposit crystal grains of SiC on a substrate, causing the crystal grains to grow into a coating, and then removing the substrate. The product is characterized by the possession of high density, a high purity, and a high structural uniformity as a material.
As formed SiC products prepared by a CVD method, chemically deposited, self-supported .beta.-SiC having an attenuation constant of approximately 20 cm.sup.-1 or less at 3 .mu.m and a chemically deposited, self-supported .beta.-SiC having an attenuation constant of approximately 20 cm.sup.-1 or less at 0.6328 .mu.m are proposed in the Japanese Patent Application Laid-Open No. 239609/1994. Whereas a CVD-formed SiC product is known to exhibit a higher degree of light transmittance as its purity increases, the CVD-formed SiC product disclosed in the above-mentioned published patent application was also of very high purity at 100% of a theoretical density, with the metallic impurity content being 5 ppm or less or preferably 3.5 ppm or less.
Because of its light transmittance, however, a CVD-formed SiC product having such a high purity could, depending on the field of application, present certain physical problems when the formed SiC product is used in various components for semiconductor manufacturing facilities such as heat treatment equipment. For instance, in the semiconductor manufacturing process there are certain process steps involving rapid thermal treatment processes (collectively termed as "RTP") such as rapid thermal annealing, rapid thermal cleaning, rapid thermal chemical vapor deposition, rapid thermal oxidation, rapid thermal nitridation, and the like. For these process steps, the use of a formed SiC product as a shielding material has been proposed so that the wafer substrate can maintain characteristics that are excellent in planar uniformity when it is subjected to rapid heating. A formed SiC product tobe used as such a shielding material is required to possess characteristics of being nontransparent to radiated heat rays (Japanese Patent Application Laid-Open No. 237789/1997).
Additionally, while accurate control of a wafer substrate is required in an RTP step, if the temperature is measured with a pyrometer, excessive light transmittance of the components such as those providing support to the wafer substrate causes undesirable disturbance light to interfere with accurate temperature control when a blackbody cavity is formed on the rear of the wafer surface to be thermally treated. To solve this problem, a method of forming the support ring supporting the wafer substrate from silicon or silicon oxide materials, and using a silicon-coated quartz material for a cylinder supporting the support ring so that the cylinder becomes nontransparent within the detecting wavelength range from the pyrometer has been proposed (Japanese Patent Application Laid-Open No. 255800/1996).
Further, to prevent light leaking from the heating element from entering the reflection cavity, a method of arranging a partition as well as a guard ring alongside the wafer, and having such a guard ring comprise a silicon material having a black or gray appearance so that the light leaking from the heating element can be absorbed has been proposed (Japanese Patent Application Laid-Open No. 341905/1994). However, the silicon-made components or silicon-coated components as disclosed in the above-mentioned Japanese Patent Application Laid-Open Nos. 341905/1994 and 255800/1996 have the shortcomings of losing their nontransparency to light due to insufficient corrosion resistance to acid cleaning performed to allow repeated usage and the resulting loss in thickness of the silicon coating.
Moreover, in a plasma etching treatment, the dummy wafer used to stabilize the wafer etching parameters is required to have a low degree of light transmittance. Whereas a dummy wafer is mounted on a carrier boat by operating a transfer robot, since the dummy wafer is identified by illumination by a laser beam, excessive light transmittance of the dummy wafer could obstruct accurate recognition of the wafer position by the robot, making it difficult to mount the dummy wafer at the desired position in the etching equipment.
The crystal form of a CVD-formed SiC product according to the prior art is .beta. type (cubic type) with a yellowish appearance, which presents a certain degree of light transmittance that is difficultto reduce. As a means of reducing the light transmittance, one method for example is to provide a surface roughening treatment so that the light is scattered at the surface thereby reducing light transmittance. According to this method, for example, a mirror-finished material with a surface roughness (Ra) conditioned to 10 nm or less has a light transmittance rate of 40 to 60% when light with a wavelength of 900 nm is applied, whereas a surface-roughened material with the Ra conditioned to 300 to 500 nm presents a light transmittance rate of 0.3 to 0.8% under the same conditions. While this arrangement appears to demonstrate a certain reduction in light transmittance, the method cannot be expected to provide a material with satisfactory nontransparency to light for a wide range of wavelengths.